1. Field of the Invention
The present invention relates to an optical waveguide element having high speed and low optical loss, and an optical modulator including the optical waveguide element.
2. Description of the Related Art
In recent years, the use of an optical integrated circuit, such as a planar optical waveguide, in a device for optical fiber communication, especially, long-distance wavelength multiplex optical fiber communication, has been considered.
PCT International Publication No. WO95/08787 discloses a single-mode silicon rib waveguide which has a PIN junction in a lateral direction and changes the earner density in the PIN junction to control refractive index.
U.S. Pat. No. 7,085,443 describes an optical modulator which is a silicon rib waveguide having a PN diode in a lateral direction, and in which the dopant density of each of P and N regions is uneven and is changed nonlinearly, the dopant density has a maximum value in a portion which is continuous to the electrode of each of the P and N regions, and the dopant density has a minimum value at the center of a waveguide mode.
Kensuke Ogawa, Kazuhiro Goi, Yong Tsong Tan, Tsung-Yang Liow, Xiaoguang Tu, Qing Fang, Guo-Qiang Lo, and Dim-Lee Kwong, “Silicon Mach-Zehnder modulator of extinction ratio beyond 10 dB at 10.0-12.5 Gbps”, Optics Express, 2011, Vol 19, No. 26, pp. B26-B31 discloses a high-speed optical modulator of 10 Gbps (gigabits/second) which uses a Mach-Zehnder waveguide having a silicon rib waveguide with a PN diode in a lateral direction as a phase modulation unit, and in which it is reported that an extinction ratio is equal to or greater than 10 dB, and optical loss is equal to or less than 10.5 dB.
An electro-optical element of PCT International Publication No. WO95/08787 uses refractive index control in a single-mode silicon waveguide. However, if a silicon waveguide with the width and thickness of a core of submicron scale is used, optical loss is increased due to optical scattering caused by lateral roughness of the waveguide core, and it is not possible to realize low optical loss, which is required for an optical integrated circuit for long-distance wavelength multiplex optical fiber communication.
Based on the distribution of the dopant density of the PN diode disclosed in U.S. Pat. No. 7,085,443, when expanding the core width in order to reduce optical loss by lateral roughness of the waveguide core, a higher order mode is generated. As a result, if a higher order mode component is superimposed on an intensity modulation signal or a phase modulation signal, an extinction ratio in intensity modulation or a Q value (Q-factor) in phase modulation is decreased, a bit error rate is increased, and an excellent optical communication system is not constructed.
In the silicon Mach-Zehnder waveguide having the PN diode disclosed in Kensuke Ogawa, Kazuhiro Goi, Yong Tsong Tan, Tsung-Yang Liow, Xiaoguang Tu, Qing Fang, Guo-Qiang Lo, and Dim-Lee Kwong, “Silicon Mach-Zehnder modulator of extinction ratio beyond 10 dB at 10.0-12.5 Gbps”, Optics Express, 2011, Vol. 19, No. 26, pp. B26-B31, it is difficult to reduce optical loss in a wavelength region including a C band (1530 to 1565 nm) and an L band (1565 to 1625 nm) more than 10 dB. Therefore, it is not possible to realize low optical loss which is required for optical components for long-distance wavelength multiplex optical fiber communication.
From the above, the present invention provides an optical waveguide element which is suitable for an optical integrated circuit, such as an optical modulator having high-speed and low optical loss, and which has a high extinction ratio in intensity modulation or a high Q value in phase modulation in an entire wavelength region for use in long-distance wavelength multiplex optical fiber communication.